1. Field of the Invention
The present invention relates to a process for producing carbon fibers (including graphite fibers) from acrylonitrile fibers. More specifically, the invention relates to a process for producing carbon fibers having excellent physical properties and satisfactory quality uniformity by the steps comprising:
A. FORMING A SPUN FIBER BUNDLE BY SPINNING AN ACRYLONITRILE POLYMER CONTAINING AT LEAST 90 MOLE % ACRYLONITRILE AND 0.01 TO 1.0 MOLE % SO.sub.3 H groups and wherein at least 5% of the SO.sub.3 H groups in the spun fiber bundle are converted to SO.sub.3 X groups, wherein X is a monovalent metal cation or ammonium ion;
B. HEAT-STRETCHING THE SPUN FIBER BUNDLE OF STEP (A) UNDER CONDITIONS WHEREIN THE COEFFICIENT OF FILAMENT SEPERABILITY OF THE SPUN FIBER BUNDLE IN SAID HEAT-STRETCHING STEP IS MAINTAINED WITHIN THE RANGE OF 1.1 TO 4.0,
SAID COEFFICIENT OF FILAMENT SEPARABILITY OF THE SPUN FIBER BUNDLE BEING: ##EQU1## AND THEN
C. CARBONIZING OR CARBONIZING AND THEN GRAPHITIZING THE FILAMENT OF STEP (B).
2. Description of the Prior Art
It is already known to obtain carbon fibers which are excellent for use in reinforcing materials, exothermic elements, heat-resisting materials, etc. by heating an acrylonitrile fiber in an oxidizing atmosphere at 200.degree. to 400.degree. C. so as to form a cyclized structure in the fiber and carbonizing the cyclized fiber in a non-oxidizing atmosphere at a higher temperature (normally above 800.degree. C.).
However, the so-called thermal stabilization step, which is the step of forming naphthyridine rings in the acrylonitrile fiber by heat-treating the fiber in an oxidizing atmosphere, is a very important step that governs the physical properties of the carbon fiber, the final product. It has been thought that this step requires a heat-treating operation under tension for a long period of time, and this has been the cause of the low productivity of carbon fibers.
A high-temperature thermal stabilization or a sharp temperature elevation is employed in order to heighten the productivity of carbon fibers. However, in either case, abrupt reactions such as intermolecular cross-linking and intramolecular cyclization will occur at a temperature about the exothermic transition point of the fiber. Accompanied with such reactions, local accumulation of heat takes place which causes an uneven reaction to produce a pitch-like or tar-like substance. Such a substance causes mutual adhesion of filaments or exerts a bad influence on the physical properties of the carbon fiber, for example a decrease in mechanical strength.
Therefore, various processes have been proposed to accelerate the cyclization reaction so that thermally stabilized fibers can be obtained in a short time, for example introduction of a cyclization-accelerating agent into acrylic fibers or introduction of nitrogen monoxide or hydrochloric acid gas into the oxidizing atmosphere. Both means are indeed effective in shortening the heat-treating time, but have not been satisfactory enough to improve the physical properties of carbon fibers. In addition, these methods involved a cost disadvantage in that an additional equipment investment is required for the disposal of the harmful gases.
As an alternative, a method has been attempted to employ, as the precursor fiber, an acrylonitrile copolymer fiber copolymerized with a carboxyl group (-COOH) containing unsaturated monomer. However, it is the present situation that these methods have not been successful to impart sufficient physical properties to the resulting carbon fibers, although heat-treating time can be shortened to some extent by the acceleration of the condensation cyclization by heating.